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Detecting equilibrium cytochrome c folding intermediates by electrospray ionisation mass spectrometry: Two partially folded forms populate the molten-globule state

Institute of Chemistry, Johannes Kepler University, A-4040 Linz, Austria

Reprint requests to: Rita Grandori, Institute of Chemistry, Johannes Kepler University, Altenbergerstrasse 69, A-4040 Linz, Austria; e-mail:rita.grandori{at}jku.at; fax: 43-732-24688747.

Abstract

Nanoelectrospray ionization mass spectrometry (nano-ESI-MS) is applied to the characterization of ferric cytochromec (cytc) conformational states under different solvent conditions. The methanol-induced molten-globule state in the pH range 2.6–3.0 is found to be populated by two distinct, partially folded conformers I_A and I_B. The more compact intermediate I_B resembles that induced by glycerol in acid-unfolded cytc. The less compact one, I_A, also can be induced by destabilization of the native structure by trifluoroethanol. I_A and I_B can be detected, in the absence of additives, around the midpoint of the acid-induced unfolding transition, providing direct evidence for involvement of equilibrium folding intermediates in cytc conformational transitions at low pH. This study shows that mass spectrometry can contribute to the characterization of molten-globule states of proteins by detection of distinct, although poorly populated, conformations involved in a dynamic equilibrium.

Keywords: Protein folding intermediates; methanol-induced molten globule; trifluoroethanol; cytochrome c acid-induced unfolding; nanoelectrospray ionization mass spectrometry

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